Studies will be conducted on the organization of DNA in the chromosomes of the fly, Drosophila and the toad, Xenopus. Attention will be paid to the control of replication of the ribosomal RNA cistrons (rDNA) in both species, using such techniques as RNA-DNA hybridization and CsCl equilibrium ultracentrifugation. Other studies will concern the biochemical structure and cytological organization of the highly repetive satellite DNA's of Drosophila.